The therapeutic activity of plant medicines is attributed to the active constituents which they contain. In some cases the intrinsic activity of natural products has been linked to specific chemical species, but in other cases the activity of the plant medicine is considered to be due to a combination of constituents acting in concert. In most plant materials the active constituent is present in varying proportions. For example, vincristine is an alkaloid present in the aerial parts of Vinca roseaea at concentrations of less than 0.1% of the dried biomass. In the case of cannabis resin, the concentration of active constituent may be more than 60% w/w of resin (hashish). Whatever the concentration in biomass, it is convenient to extract specific constituents, or produce an enriched extract, which can be then formulated into conventional dosage forms for ease of administration.
Methods of extraction which have been used to separate constituents of plant medicines and to produce enriched extracts include maceration, decoction, and extraction with aqueous and non-aqueous solvents, distillation and sublimation.
Maceration (also known as simple maceration) is defined as the extraction of a drug in a solvent with daily shaking or stirring at room temperature. After a defined period the spent, solid material is separated from the solution (macerate). Variation on the method includes agitation of the macerate and the use of temperatures up to approximately 50° C. The method was formerly used for the preparation of tinctures and extracts from low-density plant materia medica, using various strengths of ethanol as the extraction solvent.
Decoction has been used since antiquity for the preparation of traditional medicines. In traditional Chinese medicine it is customary to place the quantity of herbs required for one day's treatment into a vessel to which hot or boiling water is added. The vessel is then raised to boiling point and allowed to simmer for 1½ hours (sometimes longer). The decoction so produced is allowed to cool, separated from solid particles, and the decoction is used as the dosage form for oral administration.
Maceration and decoction rely on a short diffusion path. Inactive constituents such as lecithins, flavinoids, glycosides and sugars may act to solubilise constituents which, in the pure state, are really soluble in the solvent. A disadvantage of maceration and decoction with water or low concentrations of ethanol is that a large quantity of inert material (ballast) that does not have therapeutic value is extracted. Ballast may consist of plant cell constituents including, but not limited to, fats, waxes, carbohydrates, proteins and sugars. This may contribute to microbiological spoilage if the product is not used promptly. If dried, the extracts so produced tend to be hygroscopic and difficult to formulate. The ballast may also affect the way in which the active constituents are absorbed from the finished dosage form.
Maceration and decoction are still widely used in situations where the balance of convenience inherent in the low technology involved outweighs the lack of precision in such technology in the context of pharmaceutical production. In the case of macerates and percolates, solvents may be removed by evaporation at temperatures below 100° C. and preferably below 60° C.
A wide range of processes based on the use of non-aqueous solvents to extract the constituents from plants have been used in the prior art. The solvents employed may be miscible with water or water immiscible and vary in solvent power according to the concept of E°, which is familiar in the context of chromatography.
Traditionally, ethyl alcohol in various concentrations has been used to extract active substances from plant materials. Tinctures are alcoholic solutions produced in this way and tinctures of plant materials are described in all major pharmacopoeias. Where the final concentration of alcohol is greater than approximately 20% by volume, the tincture remains microbiologically stable and such tinctures have been widely used in compounding prescriptions. Ethanol extracts substances such as glycosides, flavinoids and alkaloid salts which are examples of classes of compound known to be biologically active. It also extracts considerable amounts of plant pigment, such as chlorophyll and carotenoids. By using higher alcoholic strengths lipid-soluble material may be extracted. Tinctures contain less ballast than macerates or decoctions, but are still complex mixtures of plant constituents. Where the presence of alcohol is not required the tincture can be evaporated to produce extracts. All pharmacopoeias contain liquid and solid extracts produced in this way.
Lipid solvents with a high E° value have been used to extract lipid soluble constituents from biomass. Examples are chlorinated solvents such as dichloromethane, chloroform and carbontetrachloride, hexane, ether, fluorinated hydrocarbons and supercritical fluid extraction with agents such as carbon dioxide.
Chlorinated solvents are no longer used commercially for extraction of plant biomass because they are themselves toxic and for pharmaceutical use the solvent must be removed. They are, however, reactive and can also result in the production of compounds which have been shown to be genotoxic—and may even be carcinogenic. Hexane and other petroleum-based solvents have a high E° value and good solvent activity, but they must be completely removed from the end product and also carry with them risk of fire and explosion.
Extraction with supercritical fluid CO2 has been used to remove active constituents from foods such as caffeine from coffee beans, and humulene and other flavours from hops (Humulus lupulus). The process allows for manipulation of E° value by variation of pressure, temperature and by the addition of accessory solvents (modifiers) such as alcohols.
A characteristic of all non-aqueous solvent methods of extraction is that they all, to a greater or lesser degree, remove lipid soluble inactive material or ballast from plant material. The ballast may consist of plant cell constituents including but not limited to fats, waxes, carbohydrates, proteins and sugars. The presence of these substances results in botanical extracts which may be hygroscopic, difficult to reduce to a powder and generally intractable as starting materials for pharmaceutical preparations. The presence of ballast may also limit the shelf-life of pharmaceutical products formulated from such extracts.
Some elements of ballast can be removed by an additional step post-extraction referred to as “winterisation”, which involves making a concentrated solution of the extract and cooling it to a temperature at which a proportion of waxes and lipid components may be precipitated, typically −20° C.
Partially purified plant extracts may be further purified by chromatographic separation. High performance liquid chromatography (HPLC) is an excellent analytical technique for determination and assay of constituents and can be used in preparative mode to produce pilot quantities of concentrated fractions and individual components, provided that the required reference standards are available. However, HPLC is subject to limitations of scale as a production technique and there remains a need for alternative methods of separation which can be used to produce production-scale quantities of plant extracts of sufficient quality for formulation into pharmaceutical dosage forms.
Distillation and sublimation have been used to separate components of plant medicines which have boiling points at or around the temperature at which water boils at atmospheric pressure (100° C.). Separation by distillation is a physical process widely used in the preparation of essential oils.
GB 635,121 describes a process for the preparation of extracts from aromatic plants by distillation with the help of a hot gas, preferably under high vacuum.
WO 99/11311 describes a vaporizer for inhalation and a method for the extraction of active ingredients from a crude natural product. This method utilizes an ascending stream of hot air, or a heated inert gas stream, to volatilize components from the natural product. The resultant vapour may then be inhaled by a user, for example to provide therapeutic benefit.
The present inventors have now determined that useful separation of certain plant constituents, which are not considered to be volatile at ambient temperatures, can be effected by extraction with a gas heated to higher temperatures than those traditionally used in distillation. Accordingly, they have developed a process for the preparation of extracts from natural products which avoids many of the disadvantages of the prior art and provides additional technical advantages, particularly in the extraction of pharmacologically active components from plant material.